STM images of a superconducting Cu-O plane and the corresponding tunneling spectrum in Bi2Sr2CaCu2O8+delta

Clear atomic images of Bi2Sr2CaCu2O8+delta cleaved surfaces have been observed at T=6 and 300 K by scanning tunneling microscopy (STM). The atomic images taken for bias voltages much lower than the Bi-O plane semiconducting gap E(g) similar to 100 meV, corresponding to the Cu-O plane, indicate that the conduction electrons exist mainly in the Cu 3d(x2-y2) and O 2p(sigma) orbitals. Tunneling spectra have been also measured in the same processes as in the Cu-O plane STM image observations at T=6 K. The low-temperature spectra are in good agreement with that in a d-wave superconductor with an anisotropic Fermi surface on which the normal density of states N(k(F)) is largest (N-max) for the maximum gap directions and decreases to similar to N-max/2 for the node directions. This, combined with the result on the N(k(F)) anisotropy in photoemission experiments, is consistent with a d(x2-y2) superconducting gap.